In the field of galvanometers there is a stringent requirement in many applications for accurate detection of the position of the moving element, without temperature drift and the like. This position information is fed back to the control circuitry for increasing the accuracy of control. There is also a need for providing such accurate detection at low cost, in a small physical volume, and in a manner requiring only low voltages. Because these requirements have not been fully met, undesirable compromises have been required.
For example, accurate detection has been achieved employing variable inductance transducers, but these are expensive, difficult to manufacture, and large physically because of the inductor elements. Accurate detection has also been achieved employing variable capacitance detectors, but with many of the same drawbacks. The present invention concerns an improved variable capacitance detector.
A known detector of the variable capacitance type for detection accuracies here concerned ("picofarad" capacitances: i.e. capacitances less than 1 and up to 10 pf capacitance) is described in the various Abbe patents (U.S. Pat. Nos., 3,694,741; 3,706,919; 3,771,051; 3,775,678; 3,775,679; 3,805,150; and 3,812,424) to which reference is made. The key to successful operation of the Abbe circuit, the use of transformers, also leads to the cost and size drawbacks inherent in such transformers. Commercial forms of this circuit have other drawbacks such as a relatively large number of components and complexity of the circuit, which add to cost and make trouble-shooting in the manufacturing process difficult. Such differential capacitance circuits have been commercially employed for detecting picofarad capacitance changes in high speed optical scanning applications, but with the drawbacks noted. Furthermore such circuits have not been used in other applications as in pen driving motors for high performance medical strip chart recorders, where cost has been an even more important criterion, and the high voltage associated with the transformer a distinct danger.
Another detector of the variable capacitance type has been used for many years in certain kinds of strip chart recorders. It employs the combination of a common base transistor circuit with a transformer. The transformer, located in the emitter leg, imposes a floating sinusoidal voltage from an oscillator upon a variable capacitor which has one plate connected via the transformer to the emitter, and the other plate connected to ground. The common base amplifier is used to enable a low voltage to be employed across the capacitor by one terminal, the emitter. The other terminal, the collector, demonstrates a high impedance. In one excursion of the sinusoid, the capacitor is charged by current from the collector, and hence the collector current is a measure of the capacitance of the variable capacitor. In the opposite excursion the capacitor is discharged via a diode to the base of the transistor.
The transformer as well as the large capacitor of this system have lead to expense, bulk and frequency-limiting inertia. Furthermore the sensitivity of this circuit is limited in that it requires a relatively high value of capacitance, e.g., 50 to 100 pf, in order to avoid detrimental temperature drift.
A prior attempt to provide a transformerless circuit for variable capacitance detection is that shown in Lion U.S. Pat. No. 3,012,192. But because the signal it produces is dependent upon a ratio involving the sum of two capacitances in the differential capacitance circuit, the circuit demontrates an inherently large common mode thermal instability. This renders it unsuitable for picofarad detection for which the present invention is intended.